But is there any risk from body-to-body massage with the following activities?
--the masseuse is naked but I'm wearing boxers, or if I'm naked but the masseuse is wearing panties?
No, herpes doesn't go through clothing. If one or both of you have underwear on, there is no risk.
--we're both naked but she only slides up and down on my buttocks area while I'm on my front (I know the area right around the anus can present a transmission risk, but what about the ass cheeks? I read that you can get herpes blisters on the buttocks, but buttocks skin is thick and not a mucuous membrane, so I'm confused)
You're correct - the skin on your butt cheeks is too thick for the virus to penetrate. You can get outbreaks there (it's not the most common area, but it's possible) but you are only infected through mucous membranes.
--face-sitting and licking/sniffing while the masseuse has her panties on (is there a risk that some vaginal fluid can seep through thin material and give me oral herpes?)
It's not spread through vaginal fluid. It's spread by direct unclothed skin to skin contact - mouth to mouth, mouth to genital, genital to genital, or genital to anal.
--handjobs (I know handjobs aren't risky, but what if the masseuse has fingered herself and then touches your genitals, is this still zero risk because herpes is skin-to-skin, or can it be transmitted in fluids?)
Hands don't spread STDs, even with fluids involved. There's too much movement, exposure to air, time, etc. It just doesn't happen.
--kissing or sucking the masseuse's breasts/nipples?
No, unless she has a sore on her breasts. It's possible, but very unlikely that someone would get herpes on their nipples. It's really very rare. Unless you see a sore, I wouldn't even give it a thought. If you see a sore anywhere on someone's body, don't put your mouth or genitals on it, and don't touch it with your body - it's a good rule to live by.